The present invention relates to a process and circuit arrangement for amplifying an input current present at the input of a feedback-coupled operational amplifier.
In processes or circuit arrangements of this kind, currents, which are, for example, produced by a photoelectric transducer element when acted on by light, are amplified such that they can be further processed without difficulty for control, indication and/or drive purposes.
Amplifier circuits for this purpose are known, e.g., from German Offenlegungsschrift No. 3,003,275 (FIG. 2a) and German Offenlegungsschrift No. 3,137,725 (FIG. 2), in which circuits logarithmic converting diodes, or transistors connected as diodes, are present in the circuit of the operational amplifier contained in the circuit and compress the input current.
An amplifier circuit of a reflex photometry device is known from German Offenlegungsschrift No. 3,230,543 (FIG. 4), in which amplifier the anode and the cathode of a transducer element are connected to the non-inverting and the inverting input, respectively, of an amplifier. The non-inverting input is connected to the collector of an NPN transistor, the collector of which is connected to the base in order to act as a diode which provides logarithmic compression. The NPN transistor has its emitter grounded, and the inverting input of the amplifier is connected to its output. The latter is connected to two further NPN transistors, which form a current mirror circuit and make possible a logarithmic expansion to twice the current supplied by the transducer element.
After the conversion, according to these known devices, of the photocurrent into a logarithmic current, the latter can be further added, for example, to the voltages corresponding to the film sensitivity. A greater current then arises on inverse delogarithmic conversion of this voltage.
However, as described, for example, in German Offenlegungsschrift No. 3,003,275, the added voltage must have a given temperature behavior, closely tied to the temperature behavior of the logarithmic conversion circuit and inverse logarithmic conversion circuit. Such a tied relationship, however, means a larger expenditure in semiconductors, amplifiers, and potentiometers, in order to be able to add the voltages.
From such an expenditure, however, there results an indirect worsening of the frequency response. Apart from this, it is sometimes desired to provide a fixed current amplification factor which cannot be varied.